System and Method of Implementing Asynchronously-Clocked Fixed-Location Devices for a Distance Determination by a Roaming Client Device

ABSTRACT

A system of asynchronously-clocked fixed-location devices and a roaming client device can be used for a distance determination of the roaming client device. The system includes a master device, at least one slave device, and any number of roaming client devices. The master device transmits a series of pulses. Once those pulses reach the client device, the client device will begin to count the number of pulses. Once those pulses reach the slave device, the slave device will transmit an acknowledgement pulse. After the client device receives the acknowledgement pulse, the client device will stop counting the pulses from the master device. The total number of pulses counted by the client device will then be used in the distance determination of the client device.

The current application claims a priority to the U.S. Provisional Patentapplication Ser. No. 61/808,117 filed on Apr. 3, 2013.

FIELD OF THE INVENTION

The present invention is in the technical field of determining adifference in distance (DiD) measurement using asynchronous elements anddistance counting that is equivalent to the more common method of timedifference of arrival (TDOA) measurements requiring synchronous elementsto determine 2-dimensional (2D) and 3-dimensional (3D) positioning. Morespecifically, the present invention is in the technical field ofdetermining the difference in distance traveled of signals sent fromasynchronous statically positioned transmitters of known position to anasynchronous mobile roaming receiver of unknown position in order todetermine the 2D or 3D position of the mobile receiver relative to thestatically positioned domain transmitters.

BACKGROUND OF THE INVENTION

TDOA is a measurement of the difference in arrival time of two signalsoriginating simultaneously (synchronously in near absolute time) fromtwo different known positions to a common unknown position in space oris the measurement in difference of arrival time of a common singlesignal of unknown origin to two different known locations in space. Thisinformation is valuable in the field of location positioning when thesignal velocities are known and/or predictable and are constant and areused to determine the distances between the three objects mentionedabove.

TDOA can make use of wired or wireless signals. Most commonly, theseconsist of light, radio, electrical voltage differentials, or soundtransported via waves, particles, pulses and packets, or any combinationthereof. TDOA is particularly useful to multilateration algorithms,where a series of TDOA values, along with known positions for the signaloriginators being timed, define a set of intersecting hyperbola in 2Dspace and hyperboloids in 3D space. Three intersecting hyperboloids canbe used to give a precise 3D coordinate location but only if theindividual TDOA values used to define the hyperboloids are accurate.

TDOA is only useful for positioning when it can be related to thedistance, or more specifically, the difference in distances, between anobject of unknown position to two objects of known position. For thisreason, the origin of signals from/to the two objects of known positionused for TDOA measurements must be brought into time synchronization toa useful resolution to make location possible. Typically, this is doneby synchronizing the objects of known position and quite often themobile object of unknown position, to a common time reference so thatthe TDOA can be directly related to the difference in distances betweenthe mentioned objects. Synchronization of these elements is not onlyvery expensive but, especially in the case where the signals aretraveling at the speed of light and being received by the mobile objectof unknown position, requires that the mobile be capable ofdifferentiating the two arrival times at a very high precision andresolution in order for the positioning information to be useful formost client aware applications.

The signal being used must also be of a consistent or predictablevelocity. Sound for instance, although generally consistent in velocityat any particular moment, travels through air at significantly differentrates depending on the current ambient temperature. However, thisvariation in velocity for sound at different ambient air temperaturescan be compensated for and predictable for short periods of time.

The primary use of TDOA and the resulting conversion to Di) is forreal-time location systems (RTLS). It can be in a server aware or clientaware location environment or possibly both. Examples of a server-awareenvironment may be the centralized tracking of expensive hospitalequipment to room level resolution or tracking a tool tagged tobroadcast a signal giving its position. In server-aware scenarios, theclient is not necessarily aware of its position. Nokia has developed aserver aware positioning system recently using Bluetooth 4.0 antennaarrays and triangulation techniques for example. One downfall of anexclusive server-aware positioning environment is that there is always alimit as to the number of clients that can be tracked.

Client-aware positioning systems are systems in which the mobile clientis aware of its position within the domain. A popular example of aclient-aware positioning system would be a global positioning service(GPS) device made by companies for automotive guidance using the GPSsatellites currently in orbit over the Earth today. The client maychoose to reveal its position to a server to form a combinedclient/server-aware positioning system, but it would not be required todetermine the client's position.

RTLS systems today are very expensive, bulky, and complicated. Theytypically require some type of system level clock synchronicity, whichmust be distributed and maintained to the mobile device. This can take alot of time for the user and is very expensive to design-in on both theserver side and the client side. In some markets, like the GPS market,this is easily offset by the scale of deployment and the number ofusers. Still in other markets these problems form a barrier to entry,especially in the very lucrative local/indoor client aware positioningmarket. Power requirements, distribution, mobile footprint, standardscompliance, device HW penetration, multipath reflection, installation,and calibration are factors that drive cost of ownership and affectmarket adoption for the client-aware local real-time location market.

Ultra-wideband (UWB) pulse radios are widely being researched at presentfor their very short pulse duration and, therefore, for their ability toboth avoid multipath reflection issues and, in moderation, avoidinterference with common narrow, wide, and broadband frequencycommunications including Wi-Fi and Bluetooth. Unfortunately, mostapplications using UWB pulses for location and ranging, still requirevery high resolution timing and experience difficulty detecting theleading edge of the pulse to get an accurate time stamp.

While TDOA of synchronized signals is commonly used as a method forfinding the DiD needed for multilateration to work, it is not the onlymethod. It is possible, as the present invention will prove, to actuallycalculate more directly, the DiD rather than an equivalent difference intime which must then be converted to a distance. The present inventionis a method for determining the DiD between an object of unknownlocation and two objects of known location. It requires no long termnetwork-wide synchronization between client devices and the positioningdomains infrastructure and seeks to significantly improve on theexisting barriers to the market mentioned in the above backgroundstatement by simplifying the method for determining an equivalentmeasurement to TDOA.

Since the method of the present invention is effectively equivalent tothe more common method of synchronous TDOA determination, it is oftenreferred to as asynchronous TDOA determination in this disclosure.

SUMMARY OF INVENTION

The method for determining a DiD to a roaming object, where all elementsin the domain are asynchronous of each other with the exception of verymomentary phase locked timing between master and slave elements only,consists of three distinct network elements: a single master pulsetransmitter, one to three slave transceivers, and any number of clientroaming devices.

Simply put, a master pulse generator sends out a steady pulse which isfirst phased locked by all slaves independently of each other. Oncephase locked, the slaves acknowledge the master pulse in phasedintervals, not to the master, but rather back to all clients in thedomain. The client devices in the mean time are simply counting andaccumulating master pulses until an acknowledgment is received by eachslave. A slave acknowledgement forces the client to latch the currentcount into the appropriate accumulator for that particular slave. Eachslave acknowledges at a different phase increment for each master pulseseries until all defined phase increments between 0 and 360 degrees areaccumulated by each client. This process improves location resolutionfor each pass.

Now in more high level detail for the present invention, the Masterpulse generator element is responsible for producing a steady stream ofvery short pulses at intermittent intervals which cover twice thedistance of the entire positioning domain giving the furthest slave timeto acknowledge back the full extent of the domain. The Master pulsegenerator is a transmitter only device and operates completelyautonomous to the other network elements. A single Master element ispresent in the domain however, multiple domains may coexist adjacent toone another.

The function of slave transceiver elements is to phase lock (PLL) to theincoming stream of master pulses and provide a single acknowledgment.However, this acknowledgement is not intended for the master but ratherfor the client elements. The slave can be provisioned to acknowledge inphase with the master (thus the PLL) or in an out of phase progressionbetween 0 and 360 degrees at particular phase intervals. One to three(or more) slaves will exist in the domain in order to achieve3-dimensional (x,y,z) Cartesian coordinate location by the roamingmobile device and relative to the master transmitter and domain slavetransceivers.

The function of the client receivers is to count master pulses until anacknowledgement is received from a slave element. When slave elementsare set to respond with multiple phase shifts, the client(s) willaccumulate these counts until all phase shifts are acknowledgedindividually by their respective corresponding slave elements. Unlimitedclient elements may operate within the domain. The raw counts can theneasily be converted to differences in distance by multiplying the phaseresolution in distance traveled between phased pulses by the number ofcounts collected less the time to phase lock over each pass.

Multiphase passes are required only to increase difference in distanceresolution since the measured resolution is limited to the distancebetween received Master pulses. Phase shifts in the slaveacknowledgement therefore serve to increase the resolution of thedifference in distance over multiple passes. This allows for lessexpensive components and helps eliminate multipath issues over eachpass.

The dependencies of the present invention include: A client element isdependent on both the master element and slave elements; the slaveelement(s) are independent of the client elements but dependent on thesingle master pulse transmitter element. The master pulse transmitterelement is independent and therefore autonomous of both the slave andclient elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a two-dimensional drawing of a typical real time locationdomain with two synchronized emitters of known position and a singlemobile device of unknown position at a system time “T=4” relative to theinitiation of TDOA determination.

FIG. 2 is a two-dimensional drawing of a typical real time locationdomain with two synchronized emitters of known position and a singlemobile device of unknown position at the end of the TDOA determinationat a system time “T=15”.

FIG. 3 is a two-dimensional drawing of a real time location domain usingthe method of the present invention containing two asynchronous emittersof known position and a single mobile device of unknown position at amobile time “T=7”.

FIG. 4 is a two-dimensional drawing of a real time location domain usingthe method of the present invention containing two asynchronous emittersof known position and a single mobile device of unknown position at amobile time “T=15” after the slave emitter has responded.

FIG. 5 is a two-dimensional drawing of a real time location domain usingthe method of the present invention containing two asynchronous emittersof known position and a single mobile device of unknown position at amobile time “T=29” at the end of the asynchronous TDOA determinationdescribed in the present invention.

FIG. 6 depicts a mathematical proof for the method of asynchronous TDOAdetermination using a pulse counter, wherein the mobile device issomewhere outside of the line from the mobile device to the slaveemitters.

FIG. 7 supports the mathematical proof for this method of asynchronousTDOA determination using a pulse counter, wherein the mobile device issomewhere on the line from the mobile device to the slave emitters.

FIG. 8 is a generic diagram of a pulse.

FIG. 9 is a generic diagram of two pulse streams that are 180 degreesout of phase with each other and have double the interval of the pulsediagram shown in FIG. 8.

FIGS. 10 through 31 are two-dimensional drawings of a three-dimensionalreal time location domain with a single master transmitter M and twoslave transceivers S1 and S2 involved in two phase (0° and 180°) DiDdeterminations relative to two roamers R1 and R2 at various progressivestates of the DiD/asynchronous TDOA determination process described inthe present invention. The counter next to the master transmitter Mindicates the number of pulses that is currently out-pulsed by themaster transmitter M. The upper number, next to the roamers R1 and R2,indicate the current count of received pulses from the mastertransmitter M relative to the roamers R1 and R2. The lower two numbersnext to the roamers R1 and R2 indicate the accumulated counts of masterpulses prior to the received acknowledgements from the slaves S1 and S2,which latch the upper counter to the lower accumulator relative to theslave acknowledgment being received.

FIG. 32 is a pulse interval diagram.

FIG. 33 is a pulse recognition diagram.

FIG. 34 is a slave acknowledgement diagram with increasing phase.

FIG. 35 is a slave acknowledgment diagram with random phase.

FIGS. 36 through 39 show the signal interaction (i.e. Signal ladderdiagram) between master, slave, and client element types over the fourpulse transmit cycles of a multi-phase DiD determination process asdescribed in the present invention with a phase increment of 90°.

DEFINITIONS AND ACRONYMS

Master Device—A fixed-position transmit-only infrastructure deviceproviding domain pulse stream and single system clock.Slave Device—A fixed-position transceiver infrastructure deviceproviding a phased acknowledgement pulse to the phase-locked masterpulse stream to all clients.Client Device—A roaming device capable of counting and accumulatingpulses from the master device until receiving an associatedacknowledgment pulse from the slave device.Real Time Location System (RTLS) Domain—Any area within reception ofboth the series of pulses from the master device and all acknowledgmentpulses necessary to get a fixed position for the client device relativeto the master device.Personal Computer (PC)—Windows based personal laptop computer with 3 ormore USB ports supporting provisioning and monitoring software for thisdevice.Pulse—A pulse for this purpose is a very short radio frequency (RF)impulse signal of 20%-40% duty cycle over the pulse transmit interval.It may be ultra-wideband or a unique set of narrow band frequencies inthe open broadcast range.Pulse Recognition—The client device must take relatively the same amountof time to recognize a pulse from either the master device or the slavedevice with less than 20-picosecond (ps) jitter. Furthermore, the clientdevice must also recognize a slave pulse and a master pulse with aslittle jitter. For instance, if a slave pulse is known to take longer torecognize than a master pulse, then an adjustment at the slave deviceshould be made to send the slave pulse earlier in order to compensatefor this. In other words, it is very important that slaveacknowledgement pulses arrive and are recognized by the client devicewith the intended phase variance relative to the phase-locked masterpulse stream.Pulse Transmit Duration (PTD)—A provisioned value in the device in termsof microseconds (μs) that governs the total duration of active pulsetransmission by a master device at a particular pulse transmit rate.This value will be available to both the slave and client devices, andthe slave and client devices will know implicitly the end of a PTD bythe extended duration between pulses. The slave and client devices donot need to time the PTD from beginning to end.Pulse Transmit Rate (PTR)—A provisioned value in the device in terms ofmegahertz (MHz) which governs the steady rate at which a master devicetransmits RF pulses. Range should be at least 100 MHz to 1 GHz in100-MHz increments.Pulse Width (PW)—The timed interval while the pulse transmitter isactive (on). Optimally, the PW is 200 ps or less, but the PW can be 1 to1.6 nanoseconds (ns), although these larger values begin to affect thePTR.PTI (Pulse Transmit Interval)—The complete transmitter on/off time (i.e.duty cycle) for a single pulse transmission. This value is inherent tothe PTR and the PW and is computed from thereof.Pulse Sleep Duration (PSD)—A provisioned value in the device in terms ofPTI counts which governs how long the transmitter is off between PTD's.This value must be at least 4 PTI's.Pulse Transmission Cycle (PTC)—The PTD plus the PSD comprise a singlePTC.Phase Increment (PI)—A provisioned value in the device in terms ofdegrees. The amount of phase adjustment repetitively and increasinglyadded to a slave acknowledgement pulse for each successive PTC. Valuesfor the PI must be 0 or a multiple of 5 degrees and must be integrallydivisible into 360 degrees.Phase Accumulation Count (PAC)—One or more PTD plus PSD, as determinedby the phase increment, needed to completely cycle through all phaseincrements from 0 to 360 degrees. This is a computed value which isequal to 360 divided by the PI. However, if the PI is left at 0, thenthe PAC is also 0. A value of 0 or 1 for PAC means only one PTD plus PSDcompletes a phase cycle. For example, a PI of 5 will render a PAC of 72,and, therefore, 72 repetitions of PTD plus PSD are needed to complete asingle PAC.Transmit Sleep Duration (TSD)—A provisioned value in the device in termsof hundreds of milliseconds (ms) which governs how long the transmitteris off between PAC.Complete Transmit Interval (CTI)—A complete PAC Cycle followed by a TSD.Each CTI will produce a single accumulated count ready to be releasedthrough the USB port to the PC SW. The accumulated count should be madeavailable immediately following the completed PAC.Acknowledgement pulse (ACK)—This is a signal generated by a slave deviceand is used to acknowledge phase lock to an incoming stream of masterpulses. The ACK must be uniquely identifiable to each particular slavedevice issuing its ACK. This could be very short pulses like the pulsestream from the master device, or it could be a longer pulse or anotherkind of wave signal. However, the ACK can take no longer to identifythan a master pulse did that initiated it. Phase lock must take placeover precisely the same number of received pulses each time a slaveissues an ACK at the beginning of a PTI.Phase Lock Loop (PLL)—The amount of time or number of received masterpulses required for the slave device to lock its internal clock to thepulse stream from the master device. This must be a static unchangingvalue and provisioned to all client devices as they enter the RTLSdomain.

DETAILED DESCRIPTION OF INVENTION

Referring to the two dimensional diagrams that are representative of theasynchronous TDOA determination domain, the domain in these diagrams arerepresented with 21 generic vertical markers (border to borderinclusive) of equal distance apart from each other, forming precisely 20equidistant spaces. This is an example for simplicity and in no way isintended as an absolute or even implied limit to the domain that thismethod may operate upon. In fact, the present invention is not limitedby physical dimensions or bounded by any specific distance in anydirection. Nor is the present invention specific to any particularsignaling, signal medium, signal rate, time or resolution.

In describing the present invention in further detail, which is themethod for determining asynchronous TDOA, the generic term for “pulse”will be used to interchangeably describe, but is not limited to, anultra wide band (UWB) pulse, a narrow band pulse, a sinusoidal wave, asquare wave, a packet, a cell, a particle, or any other signal of fixedand repeating deterministic and constant interval in the wired orwireless realm useful in providing a TDOA measurement. For example,recent acceptance and deployment of Bluetooth Low Energy (aka 4.0) intomobile devices for both Apple IOS and Android mobile phones may prove tobe the perfect platform for the present invention. Whereby thetransmission and arrival of a Bluetooth 4.0 packet would be analogous tothe term “pulse” as described in the present invention, which assumesthe arrival of such packets could be accurately timed in mobile clientdevices and repeated in the domain slave transceivers.

FIGS. 1 and 2 depict a common generalized method for synchronous TDOAdetermination in which two emitters A and B of known position aresynchronously timed to emit a signal at precisely the same time. Thesignal is used by the mobile client device R in TDOA determination.

FIG. 1 describes a snapshot in time at t=4 of a typical method forsynchronous TDOA determination using two fixed location emitters and asingle mobile roamer. FIG. 1 depicts a two dimensional configuration ofa rectangular area representing a real time positioning domain or cell.The wireless emitters A and B are positioned at opposite boundaries ofthis domain and at fixed and known locations. Emitters A and B can emitknown and distinguishable signals recognizable to the mobile clientdevice R, which is located somewhere within the present invention'sdomain range. The signal being transmitted does not inherently containany embedded or encoded information, but this does not preclude thesignal from containing such information. In certain instances, embeddingand encoding information may be quite useful but is not a requirementfor the present invention. FIG. 1 depicts a snapshot in time whereexactly four equal wavelengths or pulses of fixed intervals have beenemitted by both emitters A and B. The TDOA in this case is based off theunderstanding that emitters A and B send a signal (e.g. RF wave, lightpulse, RF pulse, sound, etc. . . . ) of the same velocity, synchronouslyin time, and radiating in a perfect circular pattern towards the mobileclient device R. The mobile client device R as depicted in FIG. 1 iscompletely unaware that a signal has been sent by either emitter A or B.

FIG. 2 describes the same domain as FIG. 1 at time t=15. Typically usinga high resolution real time clock, the mobile client device R will timethe difference between the signal arrivals from emitters A and B. Fordemonstration purposes and to clarify a comparison in methods, theclocks depicted in FIGS. 1 and 2 are in terms of even wavelengths, pulsecycles, or packets as the case may be. In reality, these clocks wouldlikely be in fractions of seconds (e.g. milliseconds, microseconds,nanoseconds, etc). The fact that these times are displayed in wholewavelengths is for visual simplicity only and for common comparison withthe new method described in the present invention.

Referring now to FIGS. 3 through 5, these figures depict a similarlocation domain in physical characteristics as the domain in FIGS. 1 and2. However, the domain shown in FIGS. 3 through 5 will use the newmethod described by the present invention.

Referring now to the present invention in detail, the mobile device R1in FIG. 3 will have at least one, but more likely two or three, internalcounters. However, the mobile client device R1 is not limited to onlythree counters. Each counter will be reserved and associated with aparticular master-slave pair in one or more domains. These counters willbe used to store the raw or observed difference in distance (DiD) valuesneeded later for multilateration or any other real time locationalgorithms to be used at a higher level. Alternatively, a single countercould be used with multiple dedicated buffers.

Referring now to the present invention in still more detail, a mastertransmitter may interact with one or more active slave emitters in anygiven location cycle either simultaneously or sequentially. The methoddescribed in this present invention describes the determination of up tothree DiD actual measurements in a single domain and therefore theinteractions of a single master device with up to three slave devices.However, this in no way limits this method or any process inferredherein to a serial action between a single or any particular number ofmaster-slave pairs. In fact, this method should work quite well withmultiple adjacent location domains each having any number of slaveinteractions and virtually limitless as to the number of roaming clientdevices being served.

Referring again to the present invention in more detail, in FIG. 3, thetwo fixed position emitters do not synchronously begin transmission oftheir respective signals as depicted in FIGS. 1 and 2. Therefore, theemitters in FIGS. 3 through 5 are labeled as a master device M and aslave device S. Additionally, the method for asynchronous DIDdetermination in the present invention does not require (nor inhibit)the mobile client device R to clock the time between receiving signalsfrom the two fixed point emitters. Rather, the mobile client device Rmerely counts a series of pulses transmitted from the master device Muntil receiving an acknowledgement pulse from the slave device S. Themaster device M will transmit the series of pulses at a predeterminedinterval. In this manner, the master device M may be maintained as theonly clock in the system thus eliminating jitter and wander betweendiverse clocks. FIG. 3 depicts the initiation of the series of pulsesthat have passed by the mobile client device R1 but not yet reached theslave device S. In FIG. 3, an outside observer would know the TDOA isequivalent to 9 pulses. However, at the time of the snapshot in FIG. 3,the mobile client device R1 only knows is the constant distance betweenthe master device M and the slave device S, which is 20, and its counterhas incremented up to 7.

Referring still to the present invention for more detail, FIG. 4 depictswhat happens just after the slave device S responds to the series ofpulses incoming from the master device M. In some configurations, theslave device S may be passive and simply reflect the signal from themaster device M, and, in other cases, the slave device S may be activeand generate the acknowledgement pulse with a known or deterministicdelay (if any) to the mobile client device R1 by maintaining a phaselock loop (PLL) with the master device M. In either case, the methodreferred to by the present invention functionally remains the same withrespect to the mobile device R1 and the master device M. The raw orobserved asynchronous DiD values collected by the present invention isconverted to the true or actual asynchronous DiD by subtracting out thedelay in the response time of the slave device S. The true or actualasynchronous DiD values may occur outside the raw asynchronous DiDdetermination function. FIG. 4 is a continuation of FIG. 3 in time andshows that the counter of the mobile client device R1 is up to 15, whichmeans that 15 pulses have been counted since the first pulse arrived atthe mobile client device R1 starting with a count of zero prior to thearrival of the first pulse. The mobile client device R1 will continue tocount until receiving an acknowledgment pulse reflected or generatedfrom the slave device S.

Referring again to the present invention for more detail, FIG. 5 is acontinuation of FIGS. 3 and 4 in time and depicts that the countedasynchronous DiD values does in fact match the observed synchronous TDOAdescribed in FIGS. 1 and 2, and the present invention does this withoutthe use of any high precision real time clocks or even a synchronousnetwork. The master device M will continue to pulse for a predefinedperiod of time, which is equivalent to the signal traversing to thefarthest reaches of the RTLS domain and back to the master device M.After predefined period of time, the master device M will cease pulsingand the asynchronous DiD determination cycle will terminate until themaster device M starts to emit the series of pulse again on itsregularly scheduled cycle. Once the acknowledgement pulse from the slavedevice S arrives at the mobile client device R1, its counter will stopincrementing as the series of pulses travel past the mobile clientdevice R1. In FIG. 5, the observed DiD can be easily determined bysimply counting the number of concentric pulse circles displayed in FIG.5 from the slave device S on the left to the mobile client device R1 andthen by subtracting the number of pulse circles from the master device Mon the right to the mobile client device R1. The observed DiD from FIGS.3 through 5 is also equivalent to the results from FIGS. 1 and 2operating in a synchronous environment.

In a specific embodiment of the present invention, FIGS. 3 through 5illustrate a step-by-step method for a single asynchronous TDOAdetermination without phased slave acknowledgements. This TDOA isbetween the master device M and the slave device S (hereinafter referredto as TDOAab). This embodiment of the present invention is described inthe following steps:

-   -   1. A master device M with a slave device S is positioned within        a designated RTLS domain. The slave device S can be either an        active regenerator or passive signal reflector. The slave device        S is not limited to either of the aforementioned devices and can        be any other device performing effectively the same        functionality.    -   2. The master device M and the associated slave device S act as        a paired unit and can interact with an unlimited number of        mobile client devices. The communication is always a        unidirectional “one to many” relationship between the        master-slave pair and the unlimited number of mobile roamers,        which are listening to the signal from the master-slave pair.    -   3. On a regular predetermined positioning cycle interval and at        a time “t(0−b)” relative to the mobile client device R1, the        master device M will begin to transmit a series of pulses at an        regular and predetermined interval that is known by the mobile        client R1. The constant distance c is between the master device        M and any particular mobile client device in terms of whole or        partial pulse lengths. The master device M will continue        transmitting pulses on its predetermined regular intervals for a        fixed time period, which is equal to the time it takes the first        pulse to reach the farthest slave device plus the time it takes        to have its acknowledge pulse travel all the way back to the        master device M. This allows the present invention to completely        cover the RTLS domain with pulses from the master device M and        worst case acknowledgement pulses from the slave device S.    -   4. At a time “t(0)” relative to the mobile client device R1, the        mobile client device R1 will receive the first pulse from the        master device M and reset its internal counter relative to the        particular master-slave pair associated with the first pulse.        The relationship between the series of pulses and its particular        master/slave pair can be determined through a number of        different methods that include, but are not limited to, pulse        carrier frequency, pulse carrier modulation, time slot, and        sequential iteration.    -   5. For each subsequent pulse received by the mobile client        device R1 prior to receiving an acknowledgement pulse from the        associated slave device S, the mobile client device R1 will        increment its counter associated with that particular        master-slave pair. While this method uses the series of pulses        as a means to increment the counter, the present invention does        not preclude using a local oscillator or digital timer to        perform the same functionality even if they are somewhat less        accurate. In fact, this derivation of the method is somewhat        useful under certain circumstances.    -   6. At a time “t(d+PLL)” relative to the mobile client device R1,        the acknowledgement pulse will arrive at the mobile client        device R1, which will terminate its counter relative to that        particular slave device or will latch a common counter into a        buffer dedicated to accumulating pulses for that particular        slave device. A distance a is the distance between the slave        device S and the mobile client device R1. A distance b is the        distance between the master device M and the mobile client        device R1. The distance c is the distance between the master        device M and the slave device S. The PLL is any delay by the        slave device in transmitting the acknowledgement pulse after        receiving the first pulse from the master salve M. The raw or        observed value of the counter, which is without the PLL delay        time, represents the distance d within the relationship        “d−c=a−b”, which is referred to in FIG. 6 as a part of a        mathematical proof. Due to the fact that the distance c is a        constant known value, inserting the distance d into the        aforementioned relationship eventually reveals that the value        for TDOAab is equal to “d−c”.

Referring now to the present invention in more detail, the raw orobserved value determined in step 6 of the method must be subtracted outthe distance in terms of pulse counts between the master device M andthe slave device S (distance c) to arrive at a useful asynchronous DiDdetermination, which has a resolution in terms of a whole pulseintervals. Any known delay in the slave device S or from the value forthe distance d must also be out the distance in terms of pulse countsbetween the master device M and the slave device S (distance c) toarrive at the useful asynchronous DiD determination. Step 6 of themethod is analogous to the mathematical proof defined below and referredto in FIG. 6. The resolution of this process is directly related to thelength of the predetermined intervals used for the series of pulses.

Referring again to the present invention for more detail, there existonly two possibilities for positioning of the mobile client device R1relative to the master device M and the slave device S. Referring toFIGS. 6 and 7, the mobile client device R1 of unknown position can existoutside the line between the master slave M and the slave device S asdepicted in FIG. 6, or the mobile client device R1 can be positionedsomewhere around the line between the master slave M and the slavedevice S as depicted in FIG. 7. The mathematical proof shows that themethod of the present invention is applicable to all cases where themobile client device R1 lies outside the line from the master device Mto the slave device S so that a triangle is formed with the distances a,b, and c as its sides. The mathematical proof is as follows:

-   -   1. Given for any triangle with the distances a, b, and c as its        sides, the length of any two sides added together must be        greater than the length of the third side.    -   2. Therefore, it is known that “c+a>b” by referring to FIG. 6.    -   3. We can then add the distance a adjacent to the distance c in        order to form a straight line of a length c+a.    -   4. If the distance b is coincidently projected upon the length        c+a, then there must be some remaining length since it is known        that “c+a>b” from steps 1 and 2. We will label that remaining        length as the distance d in order to define the relation        “d=a+c−b”.    -   5. Regardless of whether “b>c” or “b<c”, the working        relationship between distances a, b, c, and d is an equation        “b+d=c+a”.    -   6. By subtracting the distance b and the distance c from both        sides of the equation in step 5, the equation becomes “d−c=a−b”.    -   7. Using the method defined in the present invention, the value        of distance d is determined by counting pulses from the time the        first pulse of the master device M reaches the mobile client        device R1 until its acknowledgement pulse reaches the mobile        client device R1 from the slave device S. In addition to        counting pulses, the value of distance d is also determined by        subtracting out the delay in the reflected or phase locked        acknowledgement phase if there is any.    -   8. The TDOA that is required for this method is TDOAab, which is        equal to “a−b”. Since the distance c is a constant and the        distance d can be calculated via the method described in the        present invention, TDOAab substitutes “d−c” for “a−b” in order        to derive the equation “TDOAab=d−c”.

Referring again to the present invention and now to FIG. 7 for moredetail, the mobile client device R1 could be precisely on the linebetween the master device M and the slave device S or could be so closeto either the master device M and the slave device S so that the mobileclient device R1 is not within a measureable resolution to either themaster device M or the slave device S. This does not change themathematical proof described above except for the fact that thedistances a, b, and c do not form a triangle and now “c+a≧b”. This meansthat the distance b may already be coincident upon the distance c, and,thus, the distance b does not need to be projected onto the distance c(because the distance b is inherently projected upon the distance c). Inaddition, either the distance a or the distance b could be zero.Moreover, if the distance a is zero, then the distance d isautomatically zero. The mathematical proof given above is valid for anyof these scenarios. Therefore, any mobile device of unknown position,anywhere in three dimensional space, can calculate an asynchronous DiDbased on received signals from the master device M and the slave deviceS using the exact same method described in this present invention.

The objective of the present invention is to provide a small mobilefootprint and the least-cost asynchronous determination infrastructureat the highest resolution possible, which in turn enables the highestgrade of positioning algorithms. Consequently, the preferred embodimentof the present invention is described with the following. The speed oflight can be used as a consistent method for gauging distance and thedifference in distance. However, even for a signal at 2.4 GHz, a singlewavelength is approximately 5 inches long and travels through areceiving antenna in about 416 picoseconds. Therefore, even if thepulses mentioned in the method above occur on every whole 2.4 GHzwavelength interval, the present invention would require a much fastercarrier and would have to count pulses/wavelengths every 400picoseconds. From this, the present invention would still produce ±5inches of error in the asynchronous DiD. In addition, this error is onlya single dimension of our three dimensional positioning algorithm.

In order to reduce the error produced by the present invention, themethod above needs to be repeated with larger pulse intervals whileslightly shifting the phase of the pulse on each interval. This can bedone through two different techniques. One technique is to phase shiftthe series of pulses from the master device M on each successive pulsetransmission repetition or pass. However, since the algorithm for thepresent invention is asynchronous relative to all of its systemcomponents, the master device M would then need to also provide somekind of phase marker as a reference to the other system components, orthe phase shift would be meaningless. A second technique that is farsimpler is to keep the master device M ignorant of the phase shiftingand to use the slave acknowledgment to phase shift relative to a phaselock from the series of pulses from the master device M. From thistechnique, the mobile client device follows the exact same countingalgorithm for each successive phase shift without needing to know whichparticular phase increment is being used at any particular moment. Thus,the resulting signal relative to the client mobile device is exactly thesame in the second technique as the first technique under the followingconditions: each phase increment is used only once during one phasecycle and can be used in any order during that phase cycle. The secondtechnique is used in the preferred embodiment of the present invention.

Referring now to the present invention in still more detail on how toimprove the asynchronous DiD resolution, FIGS. 8 and 9 describe howpulses can be phase shifted and counted with only a slight modificationto the method described above. The preferred method shows how a seriesof pulses can be separately transmitted at two different phases withhalf as many pulses during each transmission, which allows for an equalpulse count using a slight modification to the methods described by thepresent invention. The preferred method can be used repeatedly toincrease the predetermined pulse interval so that extremely low costcomponents can be used for present invention and so that the presentinvention can be integrated into a SIM, RFID, or other Smart-Carddevice. Relative to the typical human user or mobile client device, thepreferred method can also be done with very little effect on the timetaken to complete the overall asynchronous DiD determination. Thediagrams for the present invention only demonstrate two passes, which inno way limits the number of passes that can be used during the preferredmethod. Moreover, the number of passes and the corresponding number ofpulse phase shifts are virtually limitless in order to achieve thehighest possible resolution.

Referring now to the present invention in still more detail on how toimprove the asynchronous DiD resolution by increasing predeterminedpulse intervals and phase shifting pulses, FIG. 10 depicts a pulse withtwice the interval radiated across the RTLS domain. The objective ofFIG. 10 and subsequent figures is to demonstrate how the preferredmethod counts the same number of whole pulses in two passes by phaseshifting and doubling pulse intervals as the generalized method does asingle pass without phase shifting and doubling the pulse interval. Theonly change from the generalized method is that the mobile client devicedoes not reset its counter at the beginning of each cycle as defined instep 4 of the first method. Instead, an accumulator must be added foreach acknowledgment pulse from a slave device, wherein the accumulatedcount acquired from multiple passes at different phases of a longerpulse interval stream is equivalent to the count acquired in a singlepass without phase shifting the pulse stream of a shorter pulse intervalstream.

Referring now to the present invention in more detail with the preferredmethod for asynchronous DiD determination, FIGS. 10 through 20illustrate a time progression of how the series of pulses propagatesfrom the master device M and how the acknowledgement pulses propagatefrom the slave devices within the RTLS domain during the first pass. InFIG. 10, the number near the master device M is the number of pulsesemitted by the master device M, and the numbers near the mobile clientdevices are their respective accumulated count of each pulse from themaster device M. In FIG. 11, the mobile client device R1 detects theseries of pulses from the master device M, and the accumulator for eachslave device is activated for the mobile client device. In FIG. 14, theslave device S2 has phase locked and responded with its acknowledgementpulse. Likewise in FIG. 16, the slave S1 has independently phase lockedand responded with its acknowledgement pulse. Finally in FIGS. 17 and19, both acknowledgment pulses from the slave devices S1 and S2 havebeen received by the mobile client device R1, and both accumulators aredisengaged by the mobile client device R1 with accumulated pulse countsof 17 relative to the slave device S1 and with accumulated pulse countsof 11 relative to the slave device S2. This effectively ends the firstpass for the mobile client device R1. However, the counter for themobile client device R1 continues to increment until the transmission ofthe pulses from the master device M ends for the first pass, andmonitoring for acknowledgement pulses from the slave device S1 and S2 isrearmed by the mobile client device R1.

Referring now to the present invention in more detail with the preferredmethod for asynchronous DiD determination, FIGS. 21 through 31 describea time progression of how the series of pulses propagates from themaster device M and how the acknowledgement pulses propagate from theslave devices within the RTLS domain during the second pass. The waitingtime between the first pass and the second pass (or between any numberof passes) is insignificant as long as the waiting time is short enoughsuch that the mobile client device has not moved a significant amount.If the present invention uses RF or light as a pulse carrier, then thewaiting time is negligible for slow moving objects carrying the mobileclient device such as humans on foot or objects on conveyors and canlikely be measured in microseconds. If the present invention uses soundas a pulse carrier, the waiting time becomes more critical. In FIG. 21,the second pass begins with the master device M by again transmitting aconstant stream of pulses. Similar to the first pass, each slave devicewill phase lock to the stream of pulses from the master device M andtransmit an acknowledgement pulse. However, during subsequent passesduring a phase accumulation count (PAC) cycle, the slave device willrespond out of phase, which is set at a predefined phase increment. Theslave device can respond out of phase only because the slave device hasphased locked to the series of pulses incoming from the master deviceand not because the internal clock of the slave device is synchronizedwith the master device M. The preferred method will keep the countcorrect at the accumulator of the mobile client device no matter howmany passes and phase shifts are required for the proper resolution.

In FIGS. 21 through 31, both slave devices S1 and S2 during the new passtransmit an acknowledgement pulse that is 180 degrees out of phase fromthe previous pass. The order in which the phase-incrementedacknowledgement pulses are received by the mobile client device isinconsequential to the mobile client device as long as all of the phaseincrements are used and used only once during a PAC cycle. Moreover, themobile client device is completely unaware of any phase shifting of theacknowledgement pulses. Therefore, each phase increment can be randomand independent for each slave device without affecting results receivedby the mobile client device. Consequently, the mobile client device andthe master device are kept simple with respect to the preferred method,and a slight complexity is added to the slave device.

The preferred method for determining a single or multiple asynchronousDiD's using a multi-phased approach as outlined in FIGS. 10 through 31is described in the following steps:

-   -   1. One master device M and one or more slave devices S are        positioned within a designated RTLS domain. The slave device S1        or S2 can be either an active regenerator or passive signal        reflector. The slave device is not limited to either of the        aforementioned devices and can be any other device performing        effectively the same functionality. However, for this example        and FIGS. 10 through 31, the slave device is an active        regenerator.    -   2. A master device M and its associated slave device(s) S1 and        S2 may interact with an unlimited number of mobile client        devices. The communication is always a unidirectional “one to        many” relationship between the master-slave pair and an        unlimited number of mobile roamer clients, which are listening        to the signal from the master-slave pair.    -   3. On a regular predetermined positioning cycle interval and at        a time “t(0−b)” relative to the mobile client device R1 or R2,        the master device M will begin by transmitting a continuous        series of pulses at regular and predetermined intervals known to        the mobile roamer. The distance c is defined as the distance        between the master device M and any particular mobile client        device in terms of whole or partial pulse lengths. The master        device M will continue transmitting pulses on its predetermined        regular intervals for a fixed time period, which is equal to the        time it takes the first pulse to reach the farthest slave device        plus the time it takes to have its acknowledge pulse travel all        the way back to the master device M. This allows the present        invention to completely cover the RTLS domain with pulses from        the master device M and worst case acknowledgement pulses from        the slave device S.    -   4. At a time “t(0)” relative to the mobile client device R1 or        R2, the mobile client device will receive its first pulse from        the master device M. If the accumulator is not active when the        first pulse arrives at the mobile client device, then the mobile        client device will activate and reset of its internal        accumulators and will set its common counter to a zero count.        The relationship between the series of pulses and its particular        master/slave pair can be determined through a number of        different methods that include, but are not limited to, pulse        carrier frequency, pulse carrier modulation, time slot, and        sequential iteration.    -   5. For each subsequent pulse received by the mobile client        device R1 or R2 prior to receiving an acknowledgement pulse from        the associated slave device S, the mobile client device will        increment its counter associated with that particular        master-slave pair. While this method uses the series of pulses        as a means to increment the counter, the present invention does        not preclude using a local oscillator or digital timer to        perform the same functionality even if they are somewhat less        accurate. In fact, this derivation of the method is somewhat        useful under certain circumstances.    -   6. At a time “t(c)” relative to the client mobile device R1 or        R2, where the distance c is a known constant distance between        the master device M and the slave device S1 or S2, the slave        device will phase lock to the series of pulses from the master        device M.    -   7. Following phase lock to the series of pulses for each slave        device, the slave device will send an acknowledge pulse that is        phase locked to the series of pulse from the master device M by        the next appropriate predefined phase increment. There are many        common methods that can be used by the mobile client device in        order to distinguish the acknowledgement pulse from each slave        device.    -   8. At a time “t(d+PLL)” relative to the mobile client device R1        or R2, the acknowledgement pulse from the slave device S1 or S2        will arrive at the mobile client device, and the mobile client        device will latch the current value of the counter into a        accumulator dedicated to that particular slave device. The        relationship “d=a+c−b” is previously defined in FIGS. 6 and 7,        and the PLL is previously defined as a known time for phase lock        for the slave device.    -   9. When the master device M completes transmitting the series of        pulses, the master device M will determine if the current pass        is the last pass of a predetermined number of passes based on        the phase increment used in the current pass. If the current        pass is the last pass, then the master device M will go to sleep        for the predefined transmit sleep duration (TSD) period, which        preps the present invention for the next asynchronous DiD        determination. At the same time, both the mobile client devices        R1 and R2 and the slave devices S1 and S2 will know the end of        the PAC cycle by detecting the extended TSD time delay. The        slave devices will reset their phase increments while the mobile        client devices will clear their counters and make their new DiD        values available for upper layer applications. However, if the        current pass is not the last pass, then the master device M will        go to sleep for the predefined pulse sleep duration (PSD)        period. The master device will then begin a new pass by        initiating a new pulse transmission cycle (PTC) and steps 3-9        will be repeated during the new PTC.    -   10. At the end of each PAC cycle, this 10-step process described        above will repeat indefinitely.

For the present invention, the preferred method of phase-shifting pulsescan achieve any resolution of accuracy by using smaller increments whilephase shifting and thereby increasing the number of passes. Since lighttravels at roughly 11.8 inches per nanosecond, a single pass across a1000 foot long room would only take 2000 nanoseconds or 2 microseconds.As the distance between pulses increases, the actual width of eachindividual pulse becomes less significant as does multipath issues sincethe count is level triggered. Consequently, the exact arrival time ofthe pulse is not nearly as significant as in a time-stamp based systembecause passes can be added as pulse intervals are increased. Even at1-MHz pulse intervals, a 10-GHz resolution can be obtained in 10,000passes at 2 microseconds per pass or just 20 milliseconds over a 1000foot linear space. The 10-GHz resolution would provide less than oneinch DiD accuracy, which is a significant improvement over the currenttechnology in the field. This is an extreme example but indicates thecapability of the preferred method of the present invention. Morelikely, the pulse interval should be 100 MHz or faster, which wouldreduce the DiD recognition time to just 200 microseconds instead of 20milliseconds in the same RTLS domain. Potentially, there is no reasonwhy a single master device cannot simultaneously provide pulses tomultiple slave devices.

The end result of the preferred method is a set of raw pulse counts thatis independently accumulated by each mobile client device over severalpasses and from several master-slave pairs independently. However, theset of raw pulse counts can be normalized based on the distances betweentheir associated master-slave pairs and can then be used in manydifferent ways. Another way to utilize the set of raw pulse counts is anasynchronous DiD equivalent measurement to TDOA for multilateration(also known as hyperbolic positioning) algorithms that determines aclient-aware local position. Another way to utilize the set of raw pulsecounts is to send the set of raw pulse counts to a server over a LAN/WANnetwork for server-aware processing.

The advantages of the present invention include a method forasynchronous DiD determination that is simpler than previous methods interms of system development and in terms of deployment and cost ofownership while maintaining a higher potential for location accuracy.The simplicity of the preferred method allows the present invention topenetrate the client-aware local real-time location market with a smalllow power footprint. Barriers exist in this market because of theinherent small mobile footprint, power, and diversity in today'sdevices. The device used to implement the present invention could easilybe placed inside a mobile SIM, a smart card, or an active ultra-wideband (UWB) RFID. In comparison, other companies have already integratedmuch more complicated radios and protocols into SIM, smart cards, andUWB RFID's. This includes ZigBee, Bluetooth, and Wi-Fi radios, all ofwhich are significantly more complicated than the requirements of thepreferred method and all of which have been proposed for or arecurrently being used for real time location systems (RTLS). Anotheradvantage of the method in the present invention is that this methodworks quickly and to a high resolution. Using very short pulses over a2.4G carrier, even 100-MHz pulse intervals could require only a fewmilliseconds to complete multiple DiD readings within a 1000 square footroom for every implementation device in the room without limitation.Moreover, because the pulses are spread far apart and are being countedinstead of being strictly timed, leading edge determination issues thatare needed establish a precise time stamp of arrival are eliminatedregardless of signal strength. Unlike GPS, which may require minutes oftraining in order to synchronize the clock for a client device to thestratum-1 clocks of the global GPS satellite system, no training andrelatively little time on the order of nanoseconds is required for phaselock of the slave device to the series of pulse from the master device.In addition, no time is required by the client device, which means thetime required for a client device to join or switch positioning domainsis virtually instantaneous on a PAC cycle boundary. Further, multipathon the pulses are easy to filter out and virtually inconsequential aslong as the multipath distortion does not bleed completely acrossinterval boundaries.

In the broadest sense, the present invention is a method for enabling abroad range of both server and client-aware local RTLS basedapplications at a price point, where these applications can gain broadmarket acceptance. These markets include, but are not limited to, socialmedia and marketing applications, on-site unattended location specificreal estate marketing (homes or commercial), demographic collection(conventions, malls etc), personal guidance and instruction (museums,large buildings or even efficient shopping paths), unattendedlocation-based sales and marketing (impulse, specials, e-coupons),location specific click-through information and advertising, along withall the existing current server aware markets.

While the foregoing written description of the invention enables aperson of ordinary but technically aware skill to make and use theinvention in what is considered to be the best mode at present, a personof ordinary skill will understand and appreciate the existence ofcombinations and variants, or their equivalents of the specificembodiment, method, and examples contained herein. The present inventionshould therefore not be limited by the above described embodiment andexamples or methods, but by all embodiments and methods within the scopeand spirit of the invention.

What is claimed is:
 1. A method of implementing asynchronously-clockedfixed-location devices for a distance determination by a roaming clientdevice, the method comprises the steps of: (A) providing a master deviceand an at least one slave device, wherein said slave device is separatedfrom said master device by a constant distance; (B) providing an atleast one client device located in between or around said master deviceand said slave device; (C) transmitting a series of pulses from saidmaster device, wherein said series of pulses is transmitted at apredetermined interval; (D) initiating a counter for said client device,once said client device receives a first pulse from said master device;(E) incrementing said counter as said client device receives eachsubsequent pulse from said master device; (F) transmitting anacknowledgement pulse from said slave device, once said first pulse isreceived by said slave device; (G) recording an actual value of saidcounter, once said client device receives said acknowledgement pulsefrom said slave device; and (H) calculating a time difference of arrival(TDOA) at said client device, wherein said TDOA is for said masterdevice and said slave device.
 2. The method of implementingasynchronously-clocked fixed-location devices for a distancedetermination by a roaming client device, wherein the method ofcalculating said TDOA as claimed in claim 1 comprises the steps of:providing said client device is aware of said constant distance betweensaid master device and said slave device; providing said client deviceis aware of said predetermined interval of said series of pulses;proportionately calculating a signal distance from both said actualvalue of said counter and said predetermined interval, wherein saidsignal distance is defined as a length from said master device to saidslave device and then to said client device; and calculating said TDOAwith said client device by subtracting said constant distance from saidsignal distance.
 3. The method of implementing asynchronously-clockedfixed-location devices for a distance determination by a roaming clientdevice, wherein the method of transmitting said acknowledgement pulseand calculating said TDOA as claimed in claim 1 comprises the steps of:delaying transmission of said acknowledgement pulse, once said firstpulse is received by said slave device; synchronizing said slave deviceto said master device as said slave device receives a set number ofsubsequent pulses from said master slave; transmitting saidacknowledgment pulse from said slave device, after said slave devicereceives said set number of subsequent pulses; providing said clientdevice is aware of said set number of subsequent pulses; and subtractingsaid set number of subsequent pulses from an observed value of saidcounter in order to determine said actual value of said counter.
 4. Themethod of implementing asynchronously-clocked fixed-location devices fora distance determination by a roaming client device, the method asclaimed in claim 1 comprises the steps of: providing a first slavedevice and a second slave device as said at least one slave device;uniquely identifying said acknowledgement pulse from said first slavedevice and said acknowledgement pulse from said second slave device withsaid client device; and separately recording said actual value of saidcounter for said first slave device and said actual valve of saidcounter for said second slave device.
 5. The method of implementingasynchronously-clocked fixed-location devices for a distancedetermination by a roaming client device, the method as claimed in claim1 further comprises the step of: terminating transmission of said seriesof pulses from said master device, once said master device receives saidacknowledgement pulse from each of said at least one slave device. 6.The method of implementing asynchronously-clocked fixed-location devicesfor a distance determination by a roaming client device, the method asclaimed in claim 1 further comprises the steps of: cyclically repeatingsaid steps (C) through (H) as a plurality of passes, wherein saidplurality of passes includes a first pass and an at least one subsequentpass; phase-incrementing said acknowledgement pulse with said slavedevice during each of said at least one subsequent pass; and averagingsaid signal distance for each of said plurality of passes with eachother in order to determine a more accurate signal distance at a higherspatial resolution.
 7. The method of implementing asynchronously-clockedfixed-location devices for a distance determination by a roaming clientdevice, wherein the method of implementing the plurality of passes asclaimed in claim 6 comprises the steps of: dividing 360 degrees by atotal number for said plurality of passes in order to calculate a setvalue; and phase-incrementing said acknowledgement pulse by said setvalue during each of said at least one subsequent pass.
 8. The method ofimplementing asynchronously-clocked fixed-location devices for adistance determination by a roaming client device, wherein the method ofimplementing the plurality of passes as claimed in claim 6 comprises thestep of: adjusting said predetermined interval for series of pulses inorder to set a total length of time for said plurality of passes.
 9. Themethod of implementing asynchronously-clocked fixed-location devices fora distance determination by a roaming client device, wherein the methodof implementing the plurality of passes as claimed in claim 6 comprisesthe step of: providing said client device with an accumulator for eachof said at least one slave device; incrementing said accumulator to saidactual value of said counter during said first pass, once said clientdevice receives said acknowledgement pulse from said slave device;resetting said counter for said client device after said first pass;incrementing said accumulator to said actual value of said counterduring said subsequent pass, once said client device receives saidacknowledgement pulse from said slave device; resetting said counter forsaid client device after each of said at least one subsequent pass; andtracking an overall count with said accumulator through both said firstpass and said additional pass.